Networks of breeders are offering to mutilate puppies to follow a social media trend, a BBC investigation has found.
Cutting or "cropping" ears involves removing part of the ear flap for cosmetic reasons.
One breeder told an undercover journalist it created a "striking" look for the American bully breed.
The procedure is illegal in the UK, but breeders are offering fraudulent pet passports to imply it was done abroad.
During the investigation, one breeder offered to sell a BBC Wales undercover journalist a puppy for £13,000, and spoke of clipping the dogs' ears, a practice which is banned in the UK under the Animal Welfare Act.
Paula Boyden, from Dogs Trust, said there was "no justification at all" for the practice, and it could leave the dogs with health and behavioural issues.
"Some animals then go on to develop infections or don't like people touching their ears," she said.
"We've also seen behaviour issues because dogs communicate with their ears, and without them they may have difficulty in doing that with their owners or other dogs."
Warning: Upsetting content
BBC Wales Investigates traced a number of breeders who post images of cropped-eared dogs online.
One breeder, Moheiz Adam, said it was "a shame" cropping was illegal as it gave dogs a "striking look".
He offered to sell an undercover journalist a puppy for £13,000, and said it would come with a pet passport and a microchip.
"[Another man] just takes care of it, brings back the paperwork... if questions are asked you basically [say] that's how you bought it, from an Irishman. It came from Europe and that's all you know.
"All of my dogs have been done by him and let's just say as long as you get it done at the right age, it looks 10 out of 10," he said.
During a video call, Mr Adam held up the puppy and, referring to its ear, said: "That will all come off."
Commenting on the BBC's evidence, Daniella Dos Santos, former president of the British Veterinary Association (BVA), said she was "devastated" by what it exposed.
"Cropping of ears is an illegal mutilation is done for no other reason than... cosmetic reasons and the whole conversation throughout has been about how these dogs look.
"It's all for status. There are no health benefits to these dogs at all."
Mr Adam later said that while he would "like to have dogs' ears removed" it was illegal and he had "never arranged for it to be done".
Another breeder, Joshua Harty, from Cardiff, told an undercover journalist that he, too, could arrange cropping and supply a foreign pet passport and microchip.
"To get the ears and the passport and the chip is normally around £500... my vet gets them from Turkey.
"I've taken dogs to shows in Ireland, Spain, and obviously I've had to cross borders, and I've taken these passports and I've got through every time. Never had a problem," he said.
Daniella Dos Santos said what Mr Harty was suggesting was "facilitating a crime".
"It's all completely illegal, the cropping of ears, the false passports, everything about this is a dodgy system," she said.
"Yet again, looking at these animals as commodities and money making machines rather than living sentient beings."
She added: "The reason this is happening is because there is a loophole that allows the importation of dogs with cropped ears, and that's just acting as a smokescreen clearly for it happening in the UK."
BBC Wales approached Mr Harty for comment, but he did not respond.
Social media posts
Vanessa Waddon, from dog rescue charity Hope, has been caring for pups which have been mutilated, after they were seized from an illegal breeder by Cardiff council.
The puppies were said to be worth £1,500 more after their ears were cut.
Ms Waddon said she feared demand was being driven by social media influencers posting images of cropped-eared dogs, citing footballer Marcus Rashford, Little Mix singer Leigh-Anne Pinnock and Love Island's Jack Fincham.
"Whenever celebrities post things online, people inevitably think 'oh that dog looks nice maybe I'll get one like that," she said.
"While [the celebrities] may have imported the dogs legally, there are people cropping illegally in the UK to meet demand."
BBC Wales contacted representatives for Mr Rashford, Ms Pinnock and Mr Fincham, but did not receive a response.
Restrictions on sales
The UK government has said the proposed Kept Animals Bill would restrict the importation of dogs with cropped ears and puppies under six months old.
A Welsh government spokesman said it had a five year Animal Welfare Plan for Wales to tackle illegal dog breeding, including new regulation and was monitoring developments closely.
But Ms Boyden said unless border staff check imported dogs, the law would not work.
"A ban on importation is to be welcomed, but a law is only as good as its enforcement and at the moment there is no detail on what resources will be available to check for cropped ears or if they look underage," she said.
Ms Waddon added: "We work with excellent enforcement officers, they just don't have the resources to investigate all of the cases that have been reported to them.
"You can have the best legislation in place, but unless it's being enforced, is never going to be fully effective."
BBC Wales Investigates, The hidden world of designer dog breeding, BBC One Wales 19:35 GMT
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